The present invention relates to a manufacturing method of a semiconductor film or a semiconductor substrate used when fabricating semiconductor devices, such as a semiconductor laser and a field effect transistor.
Conventionally, it has been known that a compound semiconductor including N and at least one element selected from Ga, Al, B, As, In, P and Sb in its composition (hereinafter, referred to as nitride semiconductor) has a broad bandgap from 1.9 to 6.2 eV and broad bandgap energy from the ultraviolet region to the visible region. Therefore, it is a potential semiconductor material for light emitting and light receiving devices. A typical example of the nitride semiconductor is a compound semiconductor whose composition is expressed by a general formula BxAlyGazIn1-x-y-zN, where 0xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa61, 0xe2x89xa6zxe2x89xa61, and 0xe2x89xa6x+y+zxe2x89xa61. A nitride semiconductor device is fabricated by chiefly using sapphire as a crystal growth substrate, and a light emitting diode using a GaN film formed on a sapphire substrate and a nitride semiconductor film formed on the GaN film has been commercially available to date.
However, a lattice misfit ratio between the sapphire substrate and GaN is quite high at approximately 16%, and a density of defects of the GaN film grown on the sapphire substrate reaches as high as 109 to 1010 cmxe2x88x922. Such a high density of defects causes a shortened useful life of, among others, a blue semiconductor laser fabricated on the sapphire substrate.
Here, the most ideal substrate available in manufacturing the GaN film is, naturally, a GaN substrate. However, in regard to GaN, nitrogen has an extremely high equilibrium vapor pressure in comparison with Ga, and for this reason, it is difficult to allow bulk crystal growth by the conventional pulling method or the like. Hence, there has been proposed a manufacturing method of a nitride semiconductor substrate, by which a thick GaN film is grown on a substrate made of materials other than those of the nitride semiconductor, that is, a substrate made of heterogeneous materials (for example, a sapphire substrate, a SiC substrate, a Si substrate, a GaAs substrate, etc., which is hereinafter referred to as the hetero-substrate), and the hetero-substrate is removed later.
More specifically, as is described in Reference 1 (Japanese Patent Laid-open Publication No. Hei. 10-256662), there has been proposed a manufacturing method of a nitride semiconductor substrate, by which a thick GaN film is grown on a sapphire substrate at a high temperature, and the sapphire substrate is removed later by means of grinding.
Also, as is described in Reference 2 (Michael K. Kelly et al., Japanese Journal of Applied Physics, Vol. 38, p.L217, 1999), there has been proposed another manufacturing method of a nitride semiconductor substrate, by which a thick GaN film is grown on a sapphire substrate at a high temperature, and the GaN film is separated from the sapphire substrate by irradiation of a laser beam.
Further, as is described in Reference 3 (Japanese Patent Laid-open Publication No. 2000-12900), there has been proposed still another manufacturing method of a nitride semiconductor substrate, by which a mask provided with a window is formed on a GaAs substrate so that a GaN buffer layer is formed inside the window of the mask at a low temperature, then a thick GaN epitaxial layer is grown on the GaN buffer layer by the HVPE method at a high temperature, and the GaAs substrate is removed later.
However, these conventional manufacturing methods of a semiconductor film have common inconveniences as follows.
That is, because a high temperature at or above 1000xc2x0 C. is necessary for the growth of GaN crystals, when GaN grown through epitaxial growth at a high temperature is cooled subsequently, it is affected by a difference in coefficients of thermal expansion between the GaN film thus grown and the hetero-substrate. To be more specific, when a thermal stress resulting from a difference in coefficients of thermal expansion is applied to the GaN film, a crack or a defect occurs in the GaN film or warpage occurs across the GaN film. In particular, when the GaN film is as thick as a few hundred micrometers approximately, a considerable thermal stress is applied to the GaN film, and warpage or a breaking occurs more frequently. Hence, it is difficult to cool the GaN film while securing an area substantially as large as that of the hetero-substrate.
It is therefore an object of the present invention to provide a manufacturing method of a semiconductor film capable of controlling the occurrence of a breaking or warpage while securing a nitride semiconductor film substantially as large as the substrate.
A manufacturing method of a semiconductor film of the present invention is a method, including: a step (a) of forming a first semiconductor film on a light transmitting substrate; a step (b) of separating contact between the substrate and the first semiconductor film at least at a part of an interface thereof by irradiating light in a space between the substrate and the first semiconductor film; and a step (c) of allowing growth of a second semiconductor film on the first semiconductor film while the first semiconductor film is placed on the substrate, at least the first and second semiconductor films being used as a semiconductor substrate.
According to this method, the light irradiation in the step (b) forms a thermal decomposition layer between the first semiconductor film and the substrate. When the substrate is cooled after the second semiconductor film is formed on the first semiconductor film in the step (c), the thermal decomposition layer absorbs the stress, thereby making it possible to reduce the stress applied to the second semiconductor film to the least possible.
In the step (b), at least a part of a region of the first semiconductor film adjacent to the substrate may be turned into the thermal decomposition layer. In such a case, in the step (b), the contact between the first semiconductor film and the substrate may be separated at the interface thereof almost entirely, or the contact between the first semiconductor film and the substrate may be separated only at a part of the interface thereof.
By arranging the manufacturing method so as to further include a step of forming a first mask provided with an opening portion on the substrate before the step (a), and in such a manner that, in the step (a), the first semiconductor film is grown on the substrate from a portion exposed through the opening portion of the first mask, it is possible to obtain the first semiconductor film with regions having fewer defects, such as a dislocation, above the mask.
By covering a side surface of the substrate with the first mask, it becomes easier to obtain a free-standing wafer by separating the substrate from the first and second semiconductor films.
The manufacturing method is preferably arranged in such a manner that the first mask transmits light, and in the step (b), contact between the first semiconductor film and the first mask is separated at least at a part of an interface thereof.
By arranging the manufacturing method so as to further include a step of forming, on the first semiconductor film, a second mask covering at least above the opening portion of the first mask and provided with an opening portion above the first mask after the step (a) and before the step (b), and in such a manner that, in the step (c), the second semiconductor film is grown on the first semiconductor film from a portion positioned at the opening portion of the second mask, it is possible to obtain the second semiconductor film having fewer defects as a whole.
The first mask is preferably formed from at least one film selected from an oxide film, a nitride film, an oxynitride film, and a refractory metal film.
By arranging the manufacturing method so as to further include a step of forming, on the first semiconductor film, a third mask provided with an opening portion and furnished with a function of interfering with crystal growth of the second semiconductor film after the step (a) and before the step (b), and in such a manner that, in the step (c), the second semiconductor film is grown on the first semiconductor film from a portion exposed through the opening portion of the third mask, it is possible to obtain the second semiconductor film with regions having fewer defects above the third mask.
By arranging the manufacturing method so as to further include a step of forming a fourth mask provided with an opening portion on the first semiconductor film after the step (a) and before the step (b), and a step of removing a region of the first semiconductor film positioned at the opening portion of the fourth mask by etching away the first semiconductor film with the fourth mask after the step (b) and before the step (c), and in such a manner that, in the step (c), the second semiconductor film is grown on the first semiconductor film from a portion exposed through the opening portion of the first semiconductor film, it is possible to obtain the second semiconductor film having a low density of defects as a whole.
In the step (a), a compound semiconductor film including nitrogen may be formed as the first semiconductor film.
In the step (a), a compound semiconductor film including N and at least one element selected from Ga, Al, B, As, In, P and Sb in a composition thereof may be formed as the first semiconductor film.
In the step (c), a compound semiconductor film including nitrogen may be formed as the second semiconductor film.
In the step (c), a compound semiconductor film including N and at least one element selected from Ga, Al, B, As, In, P and Sb in a composition thereof may be formed as the second semiconductor film.
In the step (a), it is preferable to se t a thickness of the first semiconductor film to 200 xcexcm or less.
In the step (b), it is preferable that a value of irradiation energy of the light is set in a range from 0.1 J/cm2 to 20 J/cm2 both inclusive.
By further including a step (d) of removing the substrate after the step (c), it is possible to obtain the second semiconductor film functioning as a free-standing wafer.